Small shells, slower growth: Experimental evidence consistent with nutrient elimination in Pagurus longicarpus

Hermit crabs lack a protective carapace and thus depend on the availability of empty gastropod shells for protection; individuals are extremely careful in their choice of shells, assessing both size and extent of damage. However, high quality shells are not always available in the natural environment. Due to the limited availability of ideal shells, hermit crabs are often forced to occupy shells that are too small for their body size, in which case the crab is not fully able to withdraw into its shell. In these instances, members of at least one species have been shown to decrease their growth rate, which may mitigate problems associated with outgrowing a disproportionately small shell. In this study, we examined a mechanism that may be used by the long-wristed hermit crab (Pagurus longicarpus) to slow their growth when confined within shells that are too small for their body size. We compared the growth rates, feeding rates, and defecation frequency between hermit crabs occupying ideally-sized shells and those occupying undersized shells. We found that hermit crabs occupying shells too small for their body mass defecated significantly more often than their counterparts in ideal-fit shells, despite members of both groups feeding at similar rates. These data suggest that hermit crabs confined within too-small shells slow growth rates by increasing nutrient elimination, effectively down-regulating their assimilation efficiency. However, the egestion rates observed in this study are a proxy for assimilation efficiency, and further research is necessary to draw specific conclusions regarding this hypothesis.